In the vacuum sealed molding process, a molding flask is fitted onto a pattern plate device to the surfaces of which a flexible film is being sucked to be in tight contact therewith. Then, the molding flask is filled with a particulate material and the upper opening of the molding flask is covered with a flexible film. Subsequently, the particulate material is hardened by the application of a negative pressure to the interior of the molding flask, and the pattern plate device is drawn apart from the molding flask. FIG. 11 shows a pattern plate device conventionally used when carrying out the above operation. In this pattern plate device, a pattern 2 is formed on a box-shaped pattern plate 1 integrally therewith, and in the pattern plate 1 there is provided a reduced pressure chamber 3 which can be connected to a vacuum pump (not shown). The interior of said reduced pressure chamber 3 is placed in communication with the surfaces of the pattern 2 by means of small-diameter holes 4, so that a sucking action is provided on the surfaces of the pattern 2 and a flexible film is sucked and placed in tight contact with said surfaces. In this pattern plate device having the above construction, it is needed to form the pattern plate 1, the pattern 2, and the reduced pressure chamber 3 as an integral unit, resulting in an increased manufacturing cost. In addition, a large amount of work and a high degree of skill are required when boring a multiplicity of the small-diameter holes 4 about 1 mm. in diameter at necessary positions on the pattern 2 including the corners thereof. These problems constitute serious disadvantages.